A number of solutions for solving the problem of determining possible slipperiness of road surfaces, in particular aimed at detecting presence or absence of water or ice on the road surface, are known. Older methods for determining the road surface condition using mechanical arrangements are known but are prone to faults and wear. A number of solutions to the problem of detecting water or ice on a road surface at a fixed position have also been suggested, but are generally not applicable to the problem of determining the road surface condition at or near a moving vehicle. For that problem, remote sensing methods using spectroscopical methods have become the dominating solution, and in particular near infrared spectroscopical methods due to the distinct spectroscopical properties of liquid and frozen water in that wavelength interval.
One of the earliest patents relating to this subject is U.S. Pat. No. 4,271,091 (1981) wherein a method of detecting ice on road surfaces by detecting an amplitude modulated light beam in the infrared region reflected from the road surface is disclosed. The method suffers from the drawback that there is no provision for separating reflectance changes occurring due to presence of ice from those occurring due to presence of water on the road surface, or due to changes in the reflectance properties of the asphalt or concrete constituting the road paving.
U.S. Pat. No. 5,218,206 (1990) discloses a method of detecting ice or water on road surfaces by detecting the reflectance of the road surface at two separate wavelengths in the infrared region. The method calculates the ratio between the two reflectances, and indicates the presence of water if the ratio exceeds a certain level or the presence of ice if the ratio falls below a certain level. If the ratio remains within an intermediate level, the method indicates that the road surface is dry, but unfortunately the ratio may also fall within this intermediate range if certain proportions of water and ice are present at the road surface.
Assuming the reflectance to be influenced by three parameters only; road paving reflectivity, effective liquid water layer thickness and effective ice layer thickness, three independent parameters need to be measured, and a number of solutions using three or more wavelengths have been suggested, i.a. U.S. Pat. No. 5,962,853, proposing detection at at least four wavelengths. Unfortunately, the absorption of light in non turbid media adheres to the Beer-Lambert law, stating that the transmission through the medium decreases exponentially with increasing layer thickness. For a detection system with finite signal dynamics, this corresponds to a very limited dynamic range in terms of layer thickness variations. To solve this problem, one may detect the presence of water or ice using several different wavelength intervals having different absorption coefficients, where detection in each interval gives a reliable indication of water or ice presence for a range of substance thicknesses. Combining results from measurements in several such intervals, an acceptable total layer thickness tolerance is achieved. Unfortunately, this implies detecting reflectances at a comparatively large number of wavelengths, necessitating a complex, and therefore expensive arrangement.
Wavelength modulation spectroscopy is a particular form of spectroscopy where the used light wavelength is modulated at a frequency f. After interfering with a substance, usually a gas, the wavelength modulation gives rise to amplitude modulation at frequencies being multiples of the wavelength modulation frequency f, and the amplitude modulated signal at one of these multiples of f is used for detection. With wavelength modulation spectroscopy it is possible to achieve higher signal to noise ratio than with other corresponding spectroscopical methods, thus making it possible to detect and measure substances in smaller concentrations than otherwise. In U.S. Pat. No. 6,356,350 a form of wavelength modulation spectroscopy is disclosed, where signals are detected at more than one of these multiples of f concurrently, and the signals received are used to calculate properties of a gas being measured, such as its concentration, temperature or pressure. The document does however not disclose a method of concurrently measuring the amount of two or more substances with overlapping spectral features, using a single detected modulated wavelength. Neither does the document disclose a method of deriving information on whether the substance being detected is turbid or not.
An object of the invention is therefore to provide a system, a method and a device which overcome the above mentioned problems with prior art surface condition detection devices.
These and other objects are attained by a system, a method and a device according to the characterising portions of the independent claims.